Heat exchanger

ABSTRACT

A heat exchanger comprising a pressure vessel ( 1 ). A plurality of serpentines ( 8 ) convey a fluid to be heated through the pressure vessel ( 1 ) in one direction. A duct ( 9 ) surrounding the serpentines ( 8 ) conveys a second fluid in the opposite direction to give up its heat to the first fluid. The duct ( 9 ) is spaced from the pressure vessel ( 1 ) and is surrounded with thermal insulation ( 23 ). An opening in the duct ( 9 ) equalises the pressure between the inside and the outside of the duct ( 9 ) which is also supported against expansion caused by the pressure inside the duct ( 9 ) exceeding the pressure outside the duct ( 9 ).

[0001] The present invention relates to a heat exchanger. The inventionis applicable to any type of heat exchanger where heat from a firstfluid stream is exchanged with heat from a second fluid stream.

[0002] The invention has particular application to a recuperator whichenables the hot gases leaving a high temperature source such as afurnace or gas turbine to heat the incoming air. Such a recuperator isused in the engine disclosed in FIG. 4 of WO 94/12785.

[0003] In this engine, a countercurrent recuperator is used to preheatcold isothermally compressed air for use in a combustion chamber usingexpanded exhaust gas from the combustion chamber. This engine can bemade to work using a conventional recuperator from gas turbinetechnology (such as the Solar Mercury 50). However, the pressure andtemperature of the exhaust gas of the engine of WO 94/12785 can begreater than in a gas turbine. For example, the exhaust gas pressure ofthe engine is 5×10⁵ Pa (5 bar) as opposed to atmospheric for a gasturbine. The air entering the recuperator will, for example, be at 2×10⁶Pa (20 bar) for a gas turbine and 1×10⁷ Pa (100 bar) or higher for theengine. The “hot” end of the recuperator (i.e. the end at which the hotexhaust gas enters and the heated air leaves) may be 750-800° C. for theengine as opposed to 500-600° C. for the gas turbine. The temperaturedifference between the “hot” and “cold” ends of the recuperator willalso be greater for the engine with the cooled exhaust gas leaving the“cold” end at a temperature of typically 250-300° C.

[0004] Therefore, although a conventional recuperator is suitable foruse with the engine, it is designed to operate with optimum efficiencyat very high flow rates and relatively low pressure. The presentinvention aims to provide a heat exchanger which operates mostefficiently at higher pressures and lower flow rates.

[0005] CH 195,866 discloses a heat exchanger having a duct inside apressure vessel and a number of pipes passing through the duct. Smallholes are provided in the wall of the duct in order to equalise thepressure across the duct. While this arrangement is effective to reduceor eliminate the stresses arising from a steady state, spatially uniformdifference in the pressure across the duct walls, it does not addressthe effects of various other stresses acting on the duct. Firstly, thereis a stress on the duct walls which arises from the steady pressure dropwithin the tube bundle and which causes a spatially non-uniform pressuredifference across the duct walls. This could be overcome by arrangingthe small holes along the length of the duct to equalise the pressuredifferences at various locations along the duct. However, this leads toa flow along the space outside of the duct which will prevent this spacefrom operating adequately as an insulator hence reducing the efficiencyof the heat exchanger. A second source of additional stress arises frompressure pulsations which may be present as a result of flow transients,which may either be part of normal operation or may be the result offault conditions. The heat exchanger of CH 195,866 is unable toaccommodate these conditions and is therefore not suitable as a modernhigh pressure heat exchanger.

[0006] According to the present invention a heat exchanger comprises apressure vessel; a first passage provided within a plurality of tubesfor a first stream in one direction through the pressure vessel; asecond passage for a second stream in the opposite direction through thevessel, the second passage comprising a duct spaced from the pressurevessel and enclosing the tubes such that heat transfer occurs across thewalls of the tubes; means to generally equalise the pressure between theinside of the duct and the space between the duct and the pressurevessel; thermal insulation between the duct and the inner surface of thepressure vessel; and a support to support the duct against expansioncaused by the pressure inside the duct exceeding the pressure outsidethe duct.

[0007] Locally, the tubes form a cross-flow heat exchanger which gives avery good heat transfer. Globally, they form a counter-current heatexchanger which allows the minimum temperature difference between thetwo flows. However, the use of the tubes with a high temperature andhigh pressure exhaust gas requires a suitable pressure vessel which isalso able to withstand the high temperatures. Materials, such as nickelalloys, which can fulfil both functions are prohibitively expensive.

[0008] For this reason, the present invention has the duct forming thesecond passage which is spaced from the pressure vessel and is alsoseparated from the pressure vessel by thermal insulation. Thus, thepressure vessel is protected from the high exhaust gas temperatures.

[0009] Further, a number of measures are provides to reduce the stresseson the duct caused by the high pressure of the stream passing throughthe duct. In particular, the means to generally equalise the pressurebetween the inside and outside of the duct ensures that the duct doesnot have to cope with anything like the full pressure of the exhaustgas. Other stresses such as those caused by the pressure drop along thetubes and by pressure pulsations within the duct are accommodated by thesupport.

[0010] The pressure vessel can therefore be designed to cope with thefull pressure of the exhaust gas at a relatively low temperature, whilethe duct must be able to withstand the maximum system temperature, butis not required to contain the full pressure of the exhaust gas and cantherefore be made of thinner material. Therefore, the vessel requiresfar less of an expensive high temperature material than would berequired in a vessel required to withstand the full system pressure andtemperature.

[0011] The means to equalise the pressure between the inside of the ductand the space between the duct and the pressure vessel may, for example,be in the form of a supply of pressurised fluid connected to the spacebetween the duct and the pressure vessel which is controlled inaccordance with the pressure within the duct so as to equalise thepressures. However, preferably, the means to equalise the pressure isone or more through holes in the wall of the duct. These simply allowthe fluid within the duct to bleed into the pressure vessel in which itis trapped in order to equalise the pressure.

[0012] If the or each through hole is provided at the cold end of theheat exchanger, this ensures that the gas bled into the pressure vesselis at its lowest possible temperature and hence will not damage thepressure vessel. Also, if the pressure vessel leaks, gas is drawn fromthe cold end of the duct thus limiting consequential damage. Further, toavoid any flow of gas along the space filled with insulation, thethrough holes are preferably all situated generally in a single planeperpendicular to the direction of flow of the streams through thevessel.

[0013] The purpose of the thermal insulation is to shield the inner wallof the pressure vessel from the high temperatures within the duct. Thus,the insulation may be provided to completely fill the space between theouter wall of the duct and the inside surface of the pressure vessel(provided that the insulation is completely gas permeable), may beprovided on the inside surface of the pressure vessel, or may beprovided by the wall of the duct itself. However, the current preferenceis for the thermal insulation to be provided against the outer wall ofthe duct.

[0014] Although the pressure is nominally equalised between the insideand the outside of the duct, it is possible that, in some applications,a non steady flow will result in pulses of increased or decreasedpressure. If there is a pressure drop across the duct, this will alsotend to stress the duct.

[0015] The support may be an internal support such as a plurality of tierods. However, such a support has to be carefully configured to avoidinterference with the tubes. The support is therefore preferablyexternal to the duct, and preferably substantially surrounds the duct.

[0016] The external support may, for example, be provided by externalreinforcing ribs. However, the presently preferred way of supporting theduct is to surround the duct with insulation held against the wall ofthe duct using the support. The support is preferably provided by one ormore cables which surround a substantial portion of the duct. The cablesmay be anchored to the inner wall of the pressure vessel or may pass allthe way around the duct in a complete circle. The or each cable ispreferably spring loaded so as to allow the duct to expand and force theinsulation outwardly, and to push the insulation back against the wallsof the ducts upon thermal contraction of the duct. This allows thesupporting of the duct to be provided by the insulation, so that theduct can be made thin-walled. It also ensures that the insulation ismaintained in close proximity with the duct thereby maintaining adequatesupport at all times.

[0017] Preferably, the or each cable is supported on a spine or a seriesof upstands projecting outwardly from a plate which extends across theouter face of the insulation. In this way, the support provided by thecable is applied across the outer face of each block, rather than simplyat its corners.

[0018] The duct preferably rests on a base within the pressure vessel.Insulation is preferably provided between the base and the duct. Thebase is preferably detachable from the pressure vessel in order tosimplify construction, assembly and maintenance of the vessel internals.In order to allow for horizontal thermal expansion of the duct withinthe pressure vessel, it is preferably supported such that it is free toexpand horizontally. It is preferable for the duct to be fixed to thebase only at the hot end to allow for such expansion.

[0019] The tubes are also susceptible to thermal expansion. This thermalexpansion can be accommodated, for example, by flexing of bends providedin the tube. This is acceptable under certain thermal loads. However, asthe thermal loads are increased, the stress on the tubes, which arealready under stress caused by the high internal pressure, may be raisedto an unacceptably high level. Any additional thermally induced stresseswill therefore reduce the creep life of the tubes. Therefore, in orderto reduce the stresses and prolong the life of the tubes, the tubes arepreferably prestressed in their cold condition. Thus, when the tubes areheated in use the thermal expansion only results in the prestressrelaxing out.

[0020] Preferably the tubes are tensioned by tie rods which pass throughthe wall of the pressure vessel.

[0021] The tubes and the duct may be made of a single material which iscapable of withstanding the maximum temperature and pressure to whichthey will be exposed. However, given the considerable variation oftemperature and pressure across the heat exchanger, the duct and/or thetubes are preferably made of a number of different parts each of adifferent material connected in series. In this way, the use of anexpensive material capable of withstanding the full system temperatureor pressure can be reduced in favour of less expensive materials.

[0022] Preferably, a header assembly comprising a number of headers isprovided within each end of the pressure vessel in order to convey fluidto and from the tubes. Preferably, a plurality of passages are providedto convey the heated fluid from the tubes and out of the pressurevessel. Using more than one pipe allows thinner walled pipes to be usedwhich are less susceptible to thermal shock during start up and shutdown. This allows the heat exchanger to be brought up to its operatingtemperature much faster than would otherwise be the case. Also, thepipes with thinner walls and smaller diameters have sufficientflexibility to take up their own thermal expansion and thus do notrequire the use of bellows or other means to compensate for the thermalexpansion. If the heated air from the recuperator is split and fed to anumber of combustor cylinders of the reciprocating engine, the number ofpipes leading from the header is preferably a multiple of the number ofcylinders in the combustor allowing the hot air to be fed to eachcylinder individually, which is far easier than attempting to split asingle flow between the various cylinders.

[0023] The header assembly at at least one end is preferably configuredsuch that each complete tube can pass by or through the header assembly.This allows for easy maintenance of the heat exchanger in which anindividual tube can be removed from the heat exchanger by detaching itfrom the header assemblies at either end and withdrawing it through oneof the header assemblies.

[0024] Each of the tubes may simply be a straight tube. However, inorder to allow for a sufficient length of tube to cause the desired heattransfer without having an unduly long pressure vessel, the tubes arepreferably tortuous. The current preference is for sinuously woundtubes. These consist of a number of straight tube sections connected by180 degree bends. The external gas flows over the straight tube sectionsin a crossflow configuration, but the succession of 180° beds providesan overall counter-current flow path of the internal air with respect tothe external gas. A further advantage of this arrangement is that it canaccommodate a substantial tube length in a compact way and in a mannerwhich provides for thermal expansion by flexing of the tube at thebends.

[0025] Each sinuously wound tube is preferably wound in a single plane,so as to produce a flat structure. The tubes are then preferablyarranged one on top of another.

[0026] In order to improve external heat transfer with the gas flowingover the tubes, a series of fins or turbulence enhancers may be providedon the outside of the tubes. The fins may be in contact with the tubesurface in order to conduct additional heat into the tube or they may bedetached, in which case they would act only as turbulence enhancers.Alternatively, internal fins or turbulence enhancers can be provided toimprove the heat transfer with air flowing inside the tubes. Since theoverall heat transfer performance is generally limited by the externalheat transfer, the greatest benefit is obtained by some form of externalfinning and/or turbulence enhancement. In particular the fins mayproject radially outwardly in a plane perpendicular to the locallongitudinal axis of the tube and may project uniformly around theentire circumference of the tube or the fins may be shaped or cropped inorder to allow close packing of neighbouring tubes.

[0027] A simpler alternative, which could be provided more cheaply inthe case of a sinuously wound tube would be to weld on fins, which wouldrun longitudinally along rather than around straight sections of thetube.

[0028] These fins could be placed only at positions, which do notobstruct neighbouring tubes. This option would not add as much surfacearea as the option of circumferential fins, but it could improve theheat transfer by increasing turbulence and directing the flow moreeffectively onto adjacent tubes. Naturally, it would be important toobtain a satisfactory balance between increased pressure loss andimproved heat transfer.

[0029] Additional enhancement of heat transfer may be achieved by theuse of internally ribbed tubing or turbulence promoters inside thetubes. For example, a turbulence promoter in the form of a spiral may beinserted into each straight length of tubing prior to bending.

[0030] Each winding of the sinuously wound tube preferably extendsacross the full width of the duct and rests on a tube support at eachside of the duct with a clearance between the winding and the wall ofthe duct. This is particularly advantageous since it allows theindividual bends to move relative to each other to accommodatedifferential thermal expansion. The tube support also facilitates theassembly the tubes and permits removal (if necessary) of individualtubes for repair or maintenance.

[0031] When a single duct is used, the tubes must extend across the fullwidth of the duct to be supported at opposing sides of the duct. Sincethe ratio of the air mass flow to the gas mass flow is fixed, it isimportant that the available flow area available to the gas, which mustflow through the gaps between adjacent tubes, is considered in relationto the flow area available to the air inside the tubes. If this is notdone, there may be excessive velocities in one fluid leading to highpressure losses in that fluid combined with low flow velocities in theother fluid leading to poor heat transfer. If the internal and externaldiameters of the tubes and the gap between adjacent tubes are alreadydecided by other factors, then it is important that the length of thestraight, crossflow section of the tubes (normally equal to the width ofthe duct) is chosen in such a way that a suitable balance of the twoflow areas is achieved. This may cause a problem if the total number oftubes leads to a rectangular duct cross-section, which is either muchtaller or much shorter in relation to its width. In either case, itmakes the cylindrical pressure vessel much larger than it should be inrelation to the number of tubes, which it contains.

[0032] If the required number of tubes is too many to be accommodated ina duct of approximately square cross-section, and other constraints donot allow sufficient adjustment of other parameters, then one option isto provide one or more tube supports spaced from the sides of the ductand extending along the duct in the direction in which the streams passthrough the vessel. This allows two or more tubes to be supported sideby side within the duct. The or each tube support would run the wholelength of the duct and extend over the full height of the duct. Anarrangement with one tube support would, for example, provide a duct ofabout twice the width and half the height, without upsetting thenecessary balance of flow areas. This is because there is now an airflow cross-section of two tubes within the width of the duct, as opposedto only one in the previous arrangement.

[0033] Instead of providing one or more tube supports down the centre ofthe duct, the same result can be achieve by providing two or more ductsections each extending in parallel in the direction in which thestreams pass through the pressure vessel. The current preference is fortwo ducts arranged side by side, thus halving the length of eachsinuously wound tube. The duct sections are more easily removed from thepressure vessel through a header assembly than a single duct.

[0034] Preferably the tubes rest on ledges fixed to the walls of theduct such that the tubes are free to slide on the ledges. This allowsfor local thermal expansion of the tubes, and helps facilitate theirremoval from the duct

[0035] An example of a heat exchanger constructed in accordance with thepresent invention will now be described with reference to theaccompanying drawings, in which:

[0036]FIG. 1 is a perspective view of the heat exchanger with parts ofthe pressure vessel and duct broken away to show the internal detail;

[0037]FIG. 2A is a side elevation of the hot end with the side wall ofthe pressure vessel removed, and some parts shown in section;

[0038]FIG. 2B is an end elevation of the hot end with the side wall ofthe pressure vessel removed, and some parts shown in section;

[0039]FIG. 2C is a plan view of the hot end with the end wall of thepressure vessel removed;

[0040]FIG. 2D is a perspective view showing the header and the tie barsonly at the hot end;

[0041]FIG. 3A is a view similar to FIG. 2A but of the cold end;

[0042]FIG. 3B is a view similar to FIG. 2B but of the cold end;

[0043]FIG. 3C is a view similar to FIG. 2C but of the cold end;

[0044]FIG. 3D is a perspective view showing the cold end header assemblyonly;

[0045]FIG. 4 is a perspective view showing a single serpentine;

[0046]FIG. 5 is a schematic cross-section through a portion of a ductand parts of four serpentines showing the mounting of the serpentineswithin the duct;

[0047]FIG. 6A is a transverse section in a vertical plane through acentral portion of the heat exchanger;

[0048]FIG. 6B is a perspective view showing a portion of the duct,insulation and base as shown in FIG. 6A;

[0049]FIG. 6C is a view similar to FIG. 6B showing an alternativesupport for the cable; and

[0050] FIGS. 7A-7H are cross-sections in a vertical plane parallel tothe main axis of the pressure vessel showing three turns of a number ofserpentines having various configurations.

[0051] The heat exchanger described is a recuperator which is designedfor use with an engine as disclosed in. FIG. 4 of WO 94/12785. Therecuperator is designed to exchange heat between a cold flow ofisothermally compressed air and a hot stream of expanded exhaust gasfrom a combustor. The heated compressed air leaving the recuperator isthen fed to the combustor.

[0052] As shown, for example in FIG. 1, the recuperator comprises apressure vessel 1 (e.g. of mild steel) inside which all other elementsare housed. The recuperator has a cold end 2 and a hot end 3. A coldcompressed air inlet 4 and a cold exhaust outlet 5 are provided at thecold end, while a hot compressed air outlet 6 and a hot exhaust inlet 7are provided at the hot end. A plurality of serpentines 8 as describedin detail below convey the compressed air from the cold end 2 to the hotend 3. A duct 9 having a substantially rectangular cross-sectionsurrounds the serpentines 8 and conveys the exhaust gas from the hot end3 to the cold end 2. The recuperator therefore acts as a counter currentheat exchanger with heat being transferred across the walls of theserpentines from the exhaust gas to the compressed air.

[0053] The pressure vessel 1 is essentially cylindrical and has twocircular end plates 10 bolted on at either end.

[0054] A hot header assembly 11, as shown in FIGS. 2A-2D, is providedwithin the duct 9 and serves to connect the plurality of serpentines 8with the outlet 6. In fact, the outlet 6 comprises twelve separate pipes6A-6L extending vertically downwardly into the duct 9. As is apparentfrom FIGS. 2A and 2B, the hot exhaust inlet 7 leads to a duct manifold12 which then splits the exhaust flow between two longitudinallyextending duct sections 9A, 9B. Six of the hot compressed air outletpipes 6A-6L lead from each duct section 9A, 9B. The structure of eachduct section is identical and only the structure of one of these will bedescribed below. Each pipe 6A-6L is connected to several of theserpentines 8. For example, as shown in FIGS. 2A and 2B the pipe 6A isconnected to eight serpentines 8A-8H. Similar connections are providedto all of the remaining pipes 6D-6L.

[0055] The header assembly 11 is held in place by six bolts 13 whichpass through the base of the duct 9 and are anchored to duct base plate14 on which the duct rests. The hot exhaust gas inlet 7 is provided witha bellows section 15 to accommodate vertical thermal expansion. Asimilar bellows section 16 is provided on a port 17 in the pressurevessel through which the hot compressed air outlet and hot exhaust inletpass from and to the pressure vessel respectively.

[0056] The cold end of the vessel will now be described with referenceto FIGS. 3A-3D. At the cold end 2 a cold header assembly 18 is providedto transfer the cold air from the cold compressed air inlet 4 to theserpentines 8. Cold compressed inlet 4 branches into four pipes 4A-4Dwhich are arranged just beyond the vertical edges of the two ductsections 9A-9B as best shown in FIG. 3B. The spacing of the pipes 4A-4Dis so as to allow individual serpentines 8 to be withdrawn from thepressure vessel by removing the end plate 10 at the cold end 2,detaching the serpentine from the pipes 4A-4D, 6A-6L to which it isfixed, and removing it axially from the pressure vessel 1 via the coldend. Each of the cold compressed air inlet pipes 4A-4D is connected to alarger number of serpentines 8 than are connected to each of the hotcompressed air outlet pipes 6A-6L. The number of pipes shown connectedin FIG. 3D has been reduced in order to clarify the drawing. However, inpractice, there will, of course, be the same number of connectionsbetween the serpentines 8 and the hot header 11, and the serpentines andthe cold header assembly 18.

[0057] The ducts 9A, 9B lead via a duct manifold 19 to cold exhaustoutlet 5. The cold header assembly 18 is not fixed to the base plate 14so as to allow for thermal expansion of the duct 9 on the base plate 14.

[0058] A single serpentine will now be described with reference to FIG.4. The serpentine is a small diameter tube which is coiled into a largenumber of sinuously wound turns by alternately bending the pipe inopposite directions. This is preferably done by cold bending the pipe inan automatic bender to a very tight radius with all bends being formedin a common plane. Each serpentine is made up of a number of sections8′, 8″, 8′″ of different materials. The first section 81 is designed forthe hottest part of the recuperator to withstand temperatures of up to770° C. The second section 8″ is designed for an intermediate part ofthe heat exchanger and can withstand temperatures of up to 650° C., andthe third section 8′″ is for the colder part of the heat exchanger andcan withstand temperatures of up to 561° C. For example, NF709 (hightemperature, exotic stainless steel) can be used at the hot end, 321stainless steel at the mid section, and 2¼Cr low alloy steel at the coldend. Each of the sections are welded together by welds 20. In fact, eachsection of a different material may in itself be made up of severalsections also welded together by welds 20.

[0059] As shown in FIG. 5, each of the serpentines are supported alongeither side by duct wall 9. The duct itself may be made up of differentmaterials, for example, Haynes 230 (expensive nickel alloy) at the hotend and 321 stainless steel at the cold end. Each duct wall is providedwith a plurality of longitudinally extending channel shaped brackets 21extending between the hot 2 and cold 3 ends. A suitable clearance isprovided between each serpentine 8 and bracket 21, and the serpentinesare not fixed to the bracket so as to allow for thermal expansion of theserpentines. This also provides for simple withdrawal of an individualserpentine 8 described above. As an alternative to the bracket 21 anglesections could be used.

[0060] The serpentines 8 may be stacked in an in-line configuration (asshown in FIG. 7A), i.e. with the turns of one serpentine directly abovethose of the one below. Alternatively, the serpentines 8 may bestaggered (as shown in FIG. 7B) with the turns of one serpentine beingoffset by half of the pitch of adjacent turns with respect to those ofthe one below.

[0061] Staggered tube arrangements such as shown in FIG. 7B increase theminimum gap between the tubes and hence reduce the gas maximum velocity,which is an important parameter determining both heat transfer andpressure loss. It is not easy to move the tubes closer together tocompensate for the increased gap because the bends and the tube supportsinterfere with each other. Thus in this situation, contrary toconventional experience, a change to staggered tubing reduces the heattransfer performance. Depending on the overall design, the reduction inpressure loss of a simple staggered tube arrangement such as that inFIG. 7B would probably not be sufficient compensation for thedegradation of heat transfer relative to that of an in-line array as inFIG. 7A.

[0062] Conventional circular fins 30 may project from the serpentines toimprove heat transfer (as shown in FIG. 7D). Alternatively, the fins 31may have a non-circular shape as shown in FIG. 7C so as not to interferewith the adjacent serpentines. This is particularly applicable toserpentines arranged in an in-line configuration where turns of adjacentserpentines will be close together.

[0063] A further alternative is to provide a single deflector 32 on eachstraight section of tubing which projects outwardly and extends axiallyalong the straight section, i.e. out of the plane of the paper as shownin FIG. 7E. These deflectors 32 can be positioned to deflect exhaust gasso that it impinges on a downstream tube. If the deflectors 32 arefastened to the tubes in such a way that there is good thermal contact,they will bring the further benefit of additional surface area and apath for heat to flow from the deflector to the tube. Alternatively,such deflectors could be provided as separate elements not attached tothe serpentines. In this case, it is envisaged that a number ofvertically aligned deflectors will be joined together on a louvre likestructure.

[0064]FIG. 7F shows a variation involving fins 33 on both sides of tubesmounted in an in-line configuration. This provides more surface areathan FIG. 7E. FIG. 7G shows a staggered tube arrangement with fins 34,which are not angled to the flow, on both sides of tubes. This gives lowpressure losses and the additional surface area would help to improvethe heat transfer of the basic staggered arrangement. FIG. 7H shows animprovement in which angled fins 35 are placed on both sides ofstaggered tubing in such a way as to increase surface area, reduce theminimum gap and provide deflection of the flow onto adjacent heattransfer surfaces. Sufficient spacing to avoid interference betweenadjacent bends and tube supports is still maintained and it is stillpossible to withdraw individual tubes for maintenance if required.

[0065] The serpentines are supported in a prestressed condition. This isdone with a system of tie rods 22. Four such tie rods 22 are provided atthe hot end as shown in FIGS. 2A, 2C and best shown in FIG. 2D. The tierods have a number of outwardly extending flanges 22A at one end whichengage with the hot compressed air outlet pipes 6A-6L. The opposite endsof the tie rods extend through end plate 10 where they are fastened bynuts 22B. Tensioning of the serpentines 6 is achieved by tightening thenuts 22B such that the tension is transmitted to the serpentines byengagement of the flanges 22A of the tie rods 22 with the hot compressedair outlet pipes 6A-6L. A similar arrangement, this time with six tierods 22 is used at the cold end 2.

[0066] The way in which the duct 9 is supported and insulated will nowbe described with reference to FIGS. 6A, 6B. The duct 9 is surrounded onall sides by blocks of insulation 23 (typically calcium silicateblocks). Additional blocks of insulation 24 are provided to cover thehot end of the duct 9 as shown in FIGS. 2A and 2C. The blocks arearranged like bricks around the duct. Two layer of blocks are used sothat the joins between blocks may be staggered. This ensures that thereis not a direct heat path through the insulation. Where blocks may pullapart from each other a packing piece of flexible ceramic woolinsulation, such as Kaowool or rockwool, may be used which will expandto fill the gap.

[0067] Other than the bottom blocks on which the duct 9 rests, theblocks of insulation 23 are each provided with a plate 25 from which aspine 26 extends across the full width of each block. The plates 25 areheld against, but not fixed to the blocks 23. At the bottom of each sideplate 25, a number of tags 25′ project towards the wall of the pressurevessel. These tags rest on a lip 14′ extending upwardly from the baseplate 14 as shown in FIG. 6B. The effect of this is that the centre ofgravity of each side plate 25 is positioned radially inwardly of thepoint of support, such that even if the cable supporting the platefails, it will still tend to be urged towards the insulation block 23 bygravitational forces. As is apparent from FIG. 6A, the spines 26 extendradially almost to the inner wall of the pressure vessel 1, and create asubstantially circular envelope other than beneath the base plate 14.Each spine is provided with a plurality of pulleys 27 which support acable 27A which surrounds all of the spines and is retained at eitherend adjacent to the base plate 14 by spring loaded support 28. Thepulleys 27 could instead be replaced by round bars.

[0068] An alternative duct support is shown in FIG. 6C. This isgenerally the same as the support of FIG. 6B and the same referencenumerals are used to denote the same components. In this arrangement,the spines 26 are replaced by a pair of upstands 26A which perform thesame function. The spring loaded support 28A is now provided midwayalong the side of the plate 25. The support 28A comprises a housing 28Bcontaining a spring 28C and a limiter 28D to limit the travel of thespring to prevent it from being damaged. When the limited 28D reachesthe end of its travel any further thermal expansion is accommodated byexpansion of the cable 27A and loading of the duct wall.

[0069] A number of plates 25 are provided along the length of the duct9. Each plate 25 may be provided with up to four cables 27A connected inparallel with associated supports to provide a degree of redundancy incase one or more of the cables should fail.

[0070] The arrangements of FIGS. 6B and 6C ensures that when the heatexchanger is in operation and the duct 9 undergoes thermal expansion,the springs in the spring loaded supports 28 expand, and the cable andspines 26 or upstands 26A apply a force across the whole width of theface of each block of insulation 23 thereby firmly supporting the duct9. The duct 9 rests on the lower insulation block 23 and is free to movewith respect to this block upon thermal expansion. When the heatexchanger is taken out of use and cooled down, the springs pull on thecable as the duct contracts, thereby ensuring that the insulationremains firmly supporting the duct.

1. A heat exchanger comprising a pressure vessel; a first passageprovided within a plurality of tubes for a first stream in one directionthrough the pressure vessel; a second passage for a second stream in theopposite direction through the vessel, the second passage comprising aduct spaced from the pressure vessel and enclosing the tubes such thatheat transfer occurs across the walls of the tubes; means to generallyequalise the pressure between the inside of the duct and the spacebetween the duct and the pressure vessel; thermal insulation between theduct and the inner surface of the pressure vessel; and a support tosupport the duct against expansion caused by the pressure inside theduct exceeding the pressure outside the duct.
 2. A heat exchangeraccording to claim 1, wherein the means to equalise the pressure is oneor more through holes in the wall of the duct.
 3. A heat exchangeraccording to claim 2, wherein the or each through hole is provided atthe cold end of the heat exchanger.
 4. A heat exchanger according toclaim 2 or claim 3, wherein a plurality of through holes are provided,the through holes all being situated generally in a single planeperpendicular to the direction of flow of the streams through thevessel.
 5. A heat exchanger according to any one of the precedingclaims, wherein the support is external to the duct.
 6. A heat exchangeraccording to claim 5, wherein the support substantially surrounds theduct.
 7. A heat exchanger according to any one of the preceding claims,wherein the thermal insulation is provided against the outer wall of theduct.
 8. A heat exchanger according to claims 6 and 7, wherein theinsulation is held against the wall of the duct by the support.
 9. Aheat exchanger according to any of the preceding claims, wherein thesupport is provided by one or more cables which surround a substantialportion of the duct.
 10. A heat exchanger according to claim 9, whereinthe or each cable is spring loaded so as to allow the duct to expand andforce the insulation outwardly, and to push the insulation back againstthe walls of the duct upon thermal contraction of the duct.
 11. A heatexchanger according to claim 9 or claim 10, wherein the or each cable issupported on a spine or series of upstands projecting outwardly from aplate which extends across the outer face of the insulation.
 12. A heatexchanger according to any one of the preceding claims, wherein the ductrests on a base and is fixed to the base only at the hot end of the heatexchanger to allow for thermal expansion.
 13. A heat exchanger accordingto any one of the preceding claims, wherein the tubes are prestressed intheir cold condition.
 14. A heat exchanger according to claim 13,wherein the tubes are tensioned by the rods which pass through the wallof the pressure vessel.
 15. A heat exchanger according to any one of thepreceding claims, wherein the duct and/or the tubes are made of a numberof different parts each of a different material connected in series. 16.A heat exchanger according to any one of the preceding claims, wherein aplurality of passages are provided to convey the heated fluid from thetubes and out of the pressure vessel.
 17. A heat exchanger according toany one of the preceding claims, wherein a header assembly comprising anumber of headers is provided within at least one end of the heatexchanger connected to the tubes and is configured such that eachcomplete tube can pass by or through the header assembly.
 18. A heatexchanger according to any one of the preceding claims, furthercomprising one or more tube supports spaced from the sides of the ductand extending along the duct in the direction in which the streams passthrough the pressure vessel.
 19. A heat exchanger according to claim 18,wherein the or each tube support is provided by two or more ductsections each extending in parallel in the direction in which thestreams pass through the pressure vessel.
 20. A heat exchanger accordingto any one of the preceding claims, wherein each tube is tortuous.
 21. Aheat exchanger according to claim 20, wherein each tube is sinuouslywound.
 22. A heat exchanger according to claim 20 or claim 21, whereineach tube. is wound in a single plane to produce a flat structure.
 23. Aheat exchanger according to claim 22, wherein a series of fins orturbulence enhancers are provided to enhance the heat exchange acrossthe walls of the tubes.
 24. A heat exchanger according to claim 21 andclaim 23, wherein the tube has straight sections separated by bends andthe fins extend longitudinally along the straight sections of the tube.25. A heat exchanger according to any one of the preceding claims,wherein the tubes rest on ledges fixed to the walls of the duct suchthat the tubes are free to slide on the ledges.